For understanding the physical condition of earthquake generation, we estimated the deviatoric stress fields and fault planes of the large earthquakes in the hypocentral area of the 2016 Kumamoto earthquake sequence, central Kyushu, Japan. We analyzed precise hypocenters and their focal mechanism tensors from 1993 to 2017 and found the complex fault planes of the largest foreshock and the mainshock during the sequence. The faults of large earthquakes could not be detected from the preceding high seismic activity in the area to the mainshock. The analyses of the deviatoric stress tensors obtained from pre- and post-sequence seismic moment tensor data revealed the temporal change in the deviatoric stress field at the vicinity of the hypocenters of the largest foreshock and the mainshock. Because of this temporal change, we found that the stress level of the deviatoric stress field in this region is the same level as the co-seismic stress change caused by the two large earthquakes. We attempted to estimate the differential stress at the region using a new method, which can be applied to the heterogeneous stress fields. The average value of the estimated differential stresses in the region was 15.4 ± 10 MPa. The rotation of the principal stress axes and the low differential stress are important information to understand the physical processes of the earthquake generation and the state of the crustal stress.
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